1. Field of the Invention
The present invention relates to an optical device of Mach-Zehnder interferometer type which can adjust a transmission characteristic by regulating the temperature of an optical path.
2. Related Background Art
An optical device of Mach-Zehnder interferometer type comprises a main optical path for guiding light from an optical input end to an optical output end, and an auxiliary optical path optically coupled to the main optical path with first and second optical couplers. Also, this optical device comprises temperature adjusting means (e.g., a heater, a Peltier device, and the like) for regulating the temperature of one or both of the main and auxiliary optical paths between the first and second optical couplers, and can adjust a transmission characteristic by this temperature regulation. Such an optical device can be used, for example, as a gain equalizer for equalizing the gain of an optical amplifier (see, for example, a literaturexe2x80x94C. R. Doerr, et al., xe2x80x9cDynamic Wavelength Equalizer in Silica Using the Single-Filtered Interferometerxe2x80x9d, IEEE Photonics Technology Letters, Vol. 11, No. 5, pp. 581-583(1999)).
However, conventional optical devices of Mach-Zehnder interferometer type have the following problem. Namely, birefringence occurs in the main optical path or auxiliary optical path due to the temperature regulation effected by the temperature adjusting means, whereby the transmissivity of light across the optical input end and the optical output end varies depending on polarization. Therefore, the optical device yields polarization-dependent loss (PDL), thereby deteriorating a transmission characteristic.
In order to overcome the following problem, it is an object of the present invention to provide an optical device having an excellent transmission characteristic by lowering PDL.
The optical device in accordance with the present invention is an optical device of Mach-Zehnder interferometer type comprising a main optical path for guiding light from an optical input end to an optical output end; an auxiliary optical path disposed beside the main optical path; first to third optical couplers for optically coupling the main and auxiliary optical paths to each other; temperature adjusting means, disposed on the main and auxiliary optical paths between the first and second optical couplers and between the second and third optical couplers, for regulating respective temperatures of the optical paths; and control means for regulating an optical loss characteristic of transmitted light within a predetermined wavelength band in the main optical path by controlling respective amounts of temperature regulation effected by the temperature adjusting means; wherein respective optical path lengths of the main and auxiliary optical paths are set such that birefringence generated upon temperature regulation effected by the temperature adjusting means becomes smaller when optical loss is greater.
The optical device control method in accordance with the present invention is a method of controlling an optical loss characteristic of the above-mentioned optical device, the method comprising the step of regulating the optical loss characteristic by controlling a difference in amounts of temperature regulation effected by respective temperature adjusting means on the main and auxiliary optical paths between the first and second optical couplers, and a difference in amounts of temperature regulation effected by respective temperature adjusting means on the main and auxiliary optical paths between the second and third optical couplers.
Such a configuration lowers birefringence in both optical paths when optical loss is large, which suppresses the dependence of transmissivity upon polarization, whereby PDL can be reduced.
It will be preferred if the respective temperature adjusting means disposed on the main and auxiliary optical paths between the first and second optical couplers have the same configuration, and the respective temperature adjusting means disposed on the main and auxiliary optical paths between the second and third optical couplers have the same configuration, since it facilitates the control.